This invention relates to a process and system for fractionating a multicomponent liquid feed mixture by utilizing selective crystallization and melting alternately in a system having a series of zones.
As a means of separation crystallization theoretically offers the advantage over other procedures such as distillation and extraction of yielding a pure component in a single stage. In practice, however, it generally is not possible to obtain a component pure by a single crystallization due to the occlusion of mother liquor by the crystal phase. Many crystallization procedures have been proposed heretofore to overcome the problem of occlusion and yield higher purity products. The following United States patents are examples of such procedures wherein the feed material flows through a crystallizing-melting system from which liquid product can be withdrawn:
______________________________________ 2,910,516 E.E. Rush October 27, 1959 3,092,673 E.E. Rush June 4, 1963 3,174,832 B.B. Bohrer March 23, 1965 3,305,320 M.E. Weech Feb. 21, 1967 ______________________________________
The present invention provides a novel procedure for obtaining a purified component from a multicomponent mixture, which utilizes alternate crystallization and melting stages in a system having a plurality of fractionating zones. The procedure is somewhat like but not strictly analogous to that known as "parametric pumping" which has been applied to separations employing a solid adsorbent such as silica gel. The parametric pumping principle has been described, for example, by N. H. Sweed and R. H. Wilhelm in IND. ENG. CHEM. FUNDAMENTALS, Vol. 8, No. 2, 221-231 (1969), and references cited therein. In parametric pumping operations an equilibrium function relating the compositions of the two phases (adsorbed and non-adsorbed) is required, but in the present procedure a precisely analogous equilibrium function does not obtain since in theory (without concern for occlusion) the preferentially crystallized component phase would have the same composition regardless of the liquid phase composition.